What does efficiency E mean in UG-34b, in other words, which joint shall be get in? flat head to shell, or seam between flat head? but actually, joint of flat to shell is always category C type; If E is the joint efficiency of seam between head, do it equal to 1 for seamless?

See the thread titled "Joint efficiency for flat head to shell joint"; currently this is just a couple threads below your question. The same question (in slightly different form) is asked and answered.

Please do note definition of joint efficiency, from Table UW-12, of any Category A weld as defined in UW-3(a)(1)!
Buy the way, where is the thread titled "Joint efficiency for flat head to shell joint"

Kimishen,

I understand that a flat head may be butt welded rather than seamless- for a very large flat head this is not unusual. In this case you have a cat. A butt weld in the flat head, and the Joint Efficiency shall be considered for the design.

DK